Big Bite does its stuff

Spectrometer helps pick pairs out of a crowd

The neutron detector was positioned vertically and sat several yards behind the proton detector. Components for BigBite’s detector packages for this experiment were from Tel Aviv University, Glasgow University, Indiana University and Hampton University. The scattering chamber was funded through a “major research initiative” grant by the University of Virginia.

Jefferson Lab's core mission is to
study the heart of ordinary matter:
the nucleus of the atom. Now Hall A
has a new magnet and detector system
designed to help physicists look
at the nucleus in a whole new light.
"BigBite" has debuted in its first
experiment at Jefferson Lab.

BigBite is a spectrometer that was
originally built in 1995 for nuclear
physics experiments at NIKHEF, the
National Institute for Nuclear Physics
and High Energy Physics in the
Netherlands. Kees de Jager, Hall A
Leader, led several research programs
at NIKHEF before coming to Jefferson
Lab. He was the program leader of
Internal Target Physics at NIKHEF,
which built and used BigBite to
explore the electric charge distribution
in the neutron and the shape of the
deuteron (a form of the hydrogen atom
containing one proton and one neutron
in its nucleus). "BigBite was used as
a spectrometer for the internal target,
which was inside the storage ring at
NIKHEF," de Jager notes.

After he came to Jefferson Lab,
de Jager arranged for BigBite to be
shipped to JLab once its planned
experiments at NIKHEF were complete.
Here, BigBite is used to complement
Hall A's two High Resolution
Spectrometers (HRS).

According to Doug Higinbotham,
one of the NIKHEF Ph.D. students
that originally used BigBite and the
Hall A staff scientist responsible for
BigBite's current experimental program,
BigBite is very useful for what
physicists call triple-coincidence
measurements. In these experiments,
physicists need to measure three particles.
Two particles are detected by
Hall A's resident High Resolution
Spectrometers, while BigBite detects
the third.

"It's a giant microscope. So
with the HRS, you see things really,
really well, but you can only see some
tiny part of the slide," he explains,
"BigBite can see the whole slide, but
it can't zoom in." However, what
BigBite lacks in precision, it makes up
with acceptance -- the size of the area
where it can detect particles. Physicists
sometimes refer to a detector's acceptance
as its bite.

"The HRS have a very small bite,
on the order of a degree; BigBite has
a huge bite -- almost 10 degrees,"
Higinbotham explains, "So for some
experiments, you'd like to have high
resolution detectors [like the HRS]
detecting interacting particles and
another large bite or large acceptance
spectrometer [BigBite] detecting recoil
particles."

JLab's first experiment using
BigBite -- Experiment E01-015
-- was inspired by an original measurement
at Brookhaven National Lab.
Scientists there were trying to measure
Short Range Correlations (SRCs). The
nucleus is built of one or more nucleons
(protons and neutrons). Studying
how nucleons interact tells physicists
something about how they're glued
together. A SRC occurs when two
nucleons interact very strongly with
one another via the strong force, the
force that glues them together in the
nucleus.

Think of the strong force as being
like a spring. It keeps nucleons at just
the right distance from one another.
Should one stray a little far, stretching
the strong force spring out, the
force works to pull the nucleons back
together. At the same time, when
nucleons push a little too close together,
squeezing the strong force spring
in, it pushes the particles apart again.
Hall A's E01-015 aims to spot
SRCs by catching the strong force
in the act of pushing nucleons apart.

According to Steve Wood, a spokesperson
on the experiment, "You really
have to understand every part of the
force between nucleons. And part
of that force is what's happening at
a short distance. The force between
nucleons gets very repulsive at short
distances." So in that moment, the
nucleons each have extra momentum,
or energy, pushing them in opposite
directions.

If an electron from CEBAF's beam
strikes one of the nucleons as they're
being pushed apart by the strong force,
the scientists can calculate that extra
momentum. The researchers are specifically
looking to hit protons that
are interacting with other protons or
neutrons in SRCs. So the HRSs were
set to detect the electron from CEBAF
and the proton it struck. Meanwhile,
BigBite and an accompanying neutron
detector were set to detect the proton
or neutron that the struck proton was
interacting with. "We detect a high
energy electron and a high energy proton,
and we use the standard Hall A
spectrometers for that. Then we look
with BigBite and the neutron detector
at the place where we expect a recoil
from the reaction from the two particles
we hit," Wood says.

Post-experimental analysis of
the data from the High Resolution
Spectrometers, BigBite and the neutron
detector will reveal what momentum
the two nucleons had at the moment
the electron struck the proton. From
that, the researchers can get a more
complete picture of SRCs, including
perhaps a better idea of how nucleons
interact via the strong force and how
often SRCs occur. Experiment E01-
015 looked at SRCs inside a carbon
nucleus. Data-taking wrapped up in
mid-April, and analysis is underway.

To date, seven experiments have
been approved for BigBite. The next
experiment is scheduled to begin installation
in December.

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